This invention relates generally to methods and apparatus for seasoning green lumber. More particularly, the invention contemplates a method and apparatus for drying wood at an accelerated rate without damage to the wood structure, which methods and apparatus also permit injection of chemicals for fireproofing or other wood treatments as part of the same process. In addition to the marked reduction in time for drying most woods, the uniformity of drying effected by the methods and apparatus of the invention improves the yield and quality of the seasoned wood produced and permits drying some particularly difficult woods (such as tanoak) which, the industry has heretofore not been able to dry satisfactorily. Also, for reasons that are not yet fully understood, initial experiments with a prototype sized model of the apparatus of the invention indicate that some types of wood can be dried with less total shrinkage than heretofore known.
As is known to those familiar with wood technology, green or freshly cut timber contains large amounts of water, ranging from as low as about 30% by weight to as high as about 900% by weight depending upon the particular species of tree and seasonal conditions at the time of cutting. (The term "by weight" is used throughout this application as referring to the weight of wood in the oven dry condition; i.e. half the weight of a green plank having 100% moisture content is the weight of the water). Some of the water lies in the cell cavities ("free water") and can be extracted without shrinkage. But some of the water is bound within the cell walls ("bound water") and as this moisture is removed, whether naturally or as part of a deliberate drying process, the wood shrinks. To complicate matters, the rate of shrinkage is not uniform, but varies substantially by direction. For example, shrinkage in the tangential direction is typically about twice as great as shrinkage in the radial direction for most woods, and shrinkage in both of these directions perpendicular to the grain is usually much greater than shrinkage along the grain.
The shrinking properties of wood are, of course, the principal reason why wood must be properly dried in advance of its use, since otherwise the wood lacks the dimensional stability necessary for nearly all applications. In addition, wood must be properly dried to prevent decay, to facilitate machining, finishing and gluing, and to improve its strength. Although the final moisture content to which wood must be dried depends upon the dimensional stability, strength and manufacturing processes required for specific applications and in some cases the relative humidity of the geographic region where used, unseasoned boards are generally considered unsuitable for most purposes.
The time required to season green timber properly without damage to the wood has long been a bottleneck in the lumber industry. In spite of the high cost of tying up massive quantities of lumber in inventory during seasoning and the obvious commercial advantages of speeding up the drying process, the industry has heretofore been unable to devise satisfactory methods for accelerating the drying of wood without damage to the wood structure. Typically, green hardwood lumber is first air dried and then further commercial dried employing a kiln drying process which generally requires upwards of six days for most wood species to reduce the water content thereof to within an acceptable range. Typical kiln drying periods for one-inch green lumber to a moisture content of 6% range from about 16 to 28 days for red oak, which is representative of some of the more refractory hardwood species; from about 11 to 15 days for white ash, a common furniture wood; and from about 2 to 7 days for Douglas-fir, which is representative of some of the soft wood species. A simple seasoning of such green lumber by air drying achieved by prolonged storage in a yard requires from about three months to about three years to dry the lumber to a moisture content in equilibrium with the surrounding environment (typically about 14%), depending on environmental conditions and the particular species of wood. Thicker planks and boards require even longer to dry since the drying period typically increases with the square of the thickness.
The principal obstacle to accelerating the drying of wood without damaging the wood structure has been the inability to control excessive and destructive temperature and moisture gradients which are formed in the drying process because of time required to transmit moisture and heat through the wood. Even when wood is air dried at room temperature, the moisture at the surface evaporates first, since, although moisture also evaporates from the surfaces of the internal cells, the high relative humidity within the cells results in a net rate of evaporation which is much slower in the interior of the wood than at the surface. Only as the water vapor in the interior gradually migrates to the surface because of the moisture gradient created in the wood does the interior eventually dry. The difference in moisture content between the surface and interior regions of the wood during the drying process lead to differences in the rate of shrinkage, thereby setting up the internal stresses which lead to splitting, checking, casehardening and similar types of seasoning defects when the stresses are sufficiently severe.
In addition to the extensive times required for present air drying and kiln drying techniques, such techniques result in a significant amount of degradation of the wood. Hardwood lumber is typically air dried to initially reduce the moisture content to a range between 20 and 30 percent before placing it in the kiln. During the air drying process, some of the wood invariably degrades because of its exposure to the elements. In addition to splitting, checking, and similar seasoning defects, this exposure often leads to stain and decay. Consequently, a significant percentage of the lumber (frequently between 5 and 15 percent) is lowered in grade before it even reaches the kiln. The amount of degradation is even worse if the lumber is not properly separated and "stickered" immediately after the log is converted into lumber.
Attempts to accelerate the drying process by external application of heat compound the problem because wood is a poor thermal conductor. The portions of the wood near the surface heat first thereby accelerating the rate of drying near the surface, but increasing the differences in moisture content and drying rate between the surface and the interior.
For this reason, conventional kiln drying techniques are limited in the amount of heat that can be applied without damage to the wood and typically employ steam or other measures to maintain a relative humidity in the surrounding air which opposes and retards the rate of drying at the surface. Elaborate schedules are typically maintained for monitoring temperature and humidity during the drying cycle to avoid developing excessive moisture gradients and destructive internal stresses. Even if necessary to avoid damage, however, use of steam to retard drying is obviously counterproductive and inefficient. Moreover, conventional kiln drying techniques still contemplate drying cycles in terms of days and weeks.
Proposals for use of vacuum drying techniques to speed evaporation and cause low temperature volatilization of moisture have not been found satisfactory. While use of vacuums can keep temperatures low enough to avoid localized charring and combustion of the wood, low temperature volatilization of the water by use of a vacuum is not by itself the answer, because it is the temperature gradients and resulting moisture gradients that lead to destructive internal stresses within the wood. Because wood is a poor thermal conductor, use of a vacuum results in a counterproductive chilling of the interior of the wood since energy is used to volatilize the moisture.
Others have proposed use of dielectric heating methods because of their known ability to supply heat internally throughout the wood to be dried. Unacceptable temperature and moisture gradients in the wood, are still created, however, because although the electric field can supply heat throughout the wood at a uniform rate, the heat is conducted away from the surface at a faster rate than in the interior. If the voltage of the electric field is not kept low, the effect is sufficiently pronounced to lead to internal charring and combustion of the wood.
Consequently, previous proposals for use of dielectric heating techniques in wood drying have typically emphasized the need to keep voltage and power input low (see, for example, Wood, U.S. Pat. No. 3,031,767) which precludes exploiting the full advantages of dielectric heating, and retards the rate of drying. To ease the temperature and moisture gradients formed even with low power dielectric heating, such proposals have also frequently provided that the process be carried out in a conventional kiln so that hot air with a maintained humidity can be circulated around the wood during the dielectric drying. Maintaining the humidity in the surrounding air is, of course, counterproductive and inefficient for the same reasons as explained in connection with conventional kiln drying. Moreover, the need to circulate hot air requires that the lumber be laboriously separated and "stickered" as in conventional kiln drying, thereby sacrificing one of the important advantages which would otherwise arise from the internal heating characteristics of dielectric heating.
Still others have experimented with heating wood internally by microwave systems, but such efforts have encountered the same problems as previously encountered with dielectric heating, as well as additional complications such as those arising from standing waves. To avoid destructive temperature gradients and internal charring of the wood, as well as damage to the generator from reflected waves, proposals for microwave drying have typically required that boards be individually dried, usually while in motion, with the radiation impinging at a suitable angle to avoid problems created by reflection. While carefully controlled procedures of this type might be useful for limited and expensive specialty applications, they are wholly unsuitable for seasoning large commercial sized loads or green lumber. Alternatively, other microwave proposals have attempted to avoid excessive temperature and moisture gradients by resorting to inefficient and counterproductive measures, such as circulating moist hot air to oppose and retard the drying operation at the surface in the same manner as conventional kiln drying techniques and previous proposals for dielectric heating.
Proposals to limit the temperature developed by dielectric heating methods by applying subatmospheric pressure have also been unsuccessful. While the internal heating effect of dielectric methods might, in theory, be carefully adjusted to compensate for the internal chilling effect of vacuum drying methods, such combination by no means ensures the uniformity of drying necessary to avoid destructive temperature and moisture gradients. By itself the application of a vacuum to dielectric heating methods serves only to limit the maximum temperature developed by lowering the temperature of volatilization, and while it is of course necessary to limit temperatures below charring levels, the principal obstacle to the development of satisfactory methods for rapid drying of wood has been the inability to limit moisture and temperature gradients, not maximum temperature. By in large the art has failed to appreciate this distinction. For example, previous proposals for dielectric drying of odd shaped specialty wood items at subatmospheric pressure fail to make any provision for uniformity in the dielectric heating. With the electric field entering the wood in various directions and concentrations the rate of heating is not uniform, and the principal advantage of dielectric heating is lost. Moreover, with specialty wood objects of varying thickness the varying distance from interior regions to the surface results in differences in temperature and moisture gradients in different directions which compound the internal stresses within the wood.
Proposals which might have effected a uniform dielectric field within the wood at subatmospheric pressure, such as by contacting a closely packed stack of lumber between parallel electrodes, have been discarded because the moisture being extracted from the wood and the high relative humidity of the surrounding reduced pressure atmosphere lead to arcing and ionization effects which cause charring of the wood. Moreover, the charred portions tend to be repeatedly attacked by successive arcs, quickly growing into a low resistance path through the wood which not only distorts the uniformity of the field, but physically damages the entire load.
Some apparently untried proposals for dielectric field heating at subatmospheric pressures have failed to recognize the arcing or ionization problem at all, and needless to say, such proposals have not been practiced satisfactorily. Others have attempted to solve the arcing problem by limiting the process to small applications with low voltage and power levels and spacing the electrodes away from the wood. The voltages employed, however, are not satisfactory for accelerated drying of commercial sized loads of lumber. Moreover, even at the limited voltages used, the spacing of the electrodes away from the wood merely results in energy wasting ionization of the gases between the electrodes and the wood causing glow discharges instead of arcs, rendering the process highly inefficient and wholly unsuitable for large scale commercial application.
In discarding previous proposals to combine dielectric and vacuum drying methods, the industry has been forced to abandon the advantages of both, since for reasons explained above, neither technique can be used satisfactorily by itself. Not only has this failure prevented the industry from progressing beyond the time consuming and expensive kiln drying techniques by which most lumber continues to be seasoned today, but it has also necessitated that fireproofing and other wood treatment be performed by separate and costly processes.
Presently, chemical wood treatments designed to enhance resistance to environmental deterioration and flame retardency are accomplished only after the lumber is partially or completely seasoned. In some cases, the application of such preservative and flame retardant agents is accomplished by first forming a solution containing a desired quantity of the material which is applied to the surface of finished lumber in a manner so as to effect an impregnation and/or coating of the outer stratum thereof. At best, the depth of impregnation from such surface applications of additive solutions is minimal necessitating that the lumber be completely processed through conventional sawmill operations before the application of such agents is performed to avoid a removal of the protective coating during subsequent milling operations. Such surface treatments of lumber invariably leave a substantially untreated core which is susceptible to deterioration when exposed to adverse climatic conditions and is susceptible to burning at the normal untreated rate once a penetration of the protective outer layer has been effected.
To overcome the foregoing problems, it has been proposed to impregnate lumber with such protective agents by applying them under pressure while confined within a suitable pressure vessel or autoclave. This technique, while providing for increased penetration of the agents is extremely time consuming and costly and has not received widespread acceptance. Previous techniques have also frequently required expensive incising procedures (cutting slots in the surfaces of the wood) to achieve sufficient chemical penetration.
By providing the first practical and successful combination of vacuum and dielectric heating techniques, the present invention permits fireproofing and other types of preservative chemicals to be impregnated deeply into the wood during the drying process without the need for separate equipment, incising, or expensive high pressure processes.